Fatigue Testing of Stents and Stent Materials

It seems we've been focusing a lot on the medical and biomaterials markets lately, but it's an industry that is growing with new techniques, standards, and more.

Traditional fatigue testing of complete stent devices is addressed by ASTM F2477 “Standard Test Methods for in vitro Pulsatile Durability Testing of Vascular Stents”, which specifies methods for fatigue of complete devices through hydrodynamic pulsation. The method involves placing complete devices into mock arteries and subjecting them to 400 million cycles of internal pressure pulsation (10 years of human heartbeats), forcing them to radially expand and contract in each cycle. The test can either be performed between pressure limits, simulating diastolic and systolic pressures; or displacement controlled, reproducing the minimum and maximum diameters that a stent would see in vivo under worse case conditions. Tests are typically performed at frequencies of up to 50 cycles per second, resulting in typical test durations in the three to six month range.

The acceptance criterion of devices is a simple pass/fail one, in that no fracture of the stent can occur during these in vitro tests for success. Many devices from varying manufacturers have undergone Pre-Market Approval (PMA) by Food Drug Administration (FDA) and have gone into clinical use. Although this traditional “Test to Success” approach of fatigue testing has not resulted in failures, the reality is that many of

these devices are fracturing in vivo.

In early 2006, the FDA and ASTM started looking at ways that could eventually improve the current durability assessment of cardiovascular devices. Initially, two working groups under the ASTM F04.30.06 Endovascular Devices Task Group were established; the first group concentrates on better understanding of the physiological conditions devices undergo in vivo and transferring this knowledge into boundary conditions for use in testing, evaluation and modelling. The second group, entitled “Fatigue to Fracture” (FtF) group, was charged with developing alternative and improved test methods for fatigue testing of cardiovascular devices.

An alternative method that is being rapidly adopted is a “Fatigue to Fracture” approach. A rudimentary technique that is more akin to aerospace testing, this methodology involves a combination of FEA modelling and in vitro testing to assess the durability of stents through established fracture mechanics techniques. These testing guidelines and standards are still under development. Several testing techniques have been developed recently that provide testing results that provide support as manufacturers submit products for regulatory approval.

To enable a representative sample of specimens to be evaluated and to reduce overall test time, multiple samples must be tested. The multi-specimen fixtures assist cardiovascular implant manufacturers to assess these long-term fatigue characteristics of nickel-titanium (Nitinol), CoCr, stainless-steel, and other stent materials and structures. It is important that each specimen station feature a fatigue-rated load cell, precision alignment adjustment, and applicable grips for the material or structure undergoing test. The specimens should be tested in vitro at body temperatures and results should include trend monitoring of forces to determine each specimen fracture.